This application is related to commonly assigned U.S. Pat. application Ser. No. 08/415,967, (RD-24,320) filed Apr. 3, 1995, of AR Duggal, filed concurrently herewith, the disclosure of which is hereby incorporated by reference.
This invention relates generally to imaging of acoustic energy, more particularly to laser-based ultrasound detection equipment and most particularly to ultrasonic/vibration detection using Polarization Beating in a microchip laser.
Ultrasound equipment is commonly used in medical imaging and for non-destructive evaluation of materials. Ultrasound analysis involves the propagation of energy through solids, liquids, and gases as acoustic waves; typically a pulse of acoustic energy is applied to an object to be imaged and reflected waves of the acoustic pulse are detected and processed for imaging and analysis. The spatial relationship and amplitude of the reflected waves provide information as to the location and nature of structures that reflected the acoustic energy in the object being analyzed.
Piezoelectric transducers are frequently used to generate ultrasound pulses transmitted into the object to be analyzed and to detect reflected waves received at the transducer. Piezoelectric devices require extensive electrical cabling which places practical limits on the number of pixels that can be placed in a transducer array, which in turn limits the resolution of the array.
Optical techniques have also been used for generation and detection of acoustic waves in ultrasound imaging. For example, energy from a laser beam focused on the surface of an object to be examined can generate an acoustic pulse in the object. The return pulse of acoustic energy is typically detected optically through the use of interferometry. A review of such techniques is provided in the book Laser Ultrasonics--Techniques and applications by C. B. Scruby and L. E. Drain (IOP Publishing Ltd 1990), which is incorporated herein by reference. Noninterferometric techniques of optical detection of ultrasound include the knife-edge and surface-grating techniques and techniques based on reflectivity and light filters. See "Optical Detection of Ultrasound" by J. P. Monchalin, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, UFFC-33, September 1986, which is incorporated herein by reference. These laser-based methods of ultrasound detection are much less sensitive, by several orders of magnitude, than conventional piezoelectric-based methods.
Another laser-based method for detecting sound waves has been suggested in the article "Laser Hydrophone" by Y. A. Bykovskii et al., in Soy. Phys. Acoust. 34, p. 204, March 1988. In the Bykovskii et al. optical hydrophone, movement of the hydrophone membrane varies the power and/or the phase of a semiconductor laser in the hydrophone to generate changes in the amplitude of an optical signal. The Bykovskii sonar hydrophone is relatively inefficient and thus has low sensitivity.
In a more recent approach to the use of an optical transducer assembly for ultrasound applications, as disclosed in U.S. Pat. No. 5,353,262, assigned to the assignee of the present application, the disclosure of which is herein incorporated by reference, an optical transducer assembly includes a transducer housing and a signal laser mounted in the transducer housing. The optical frequency generated by the signal laser is modulable (that is, adapted to or capable of being modulated) in correspondence with acoustic energy incident on the transducer assembly. The signal laser comprises an optical cavity in which a lasing medium is disposed, and first and second reflectors that are disposed at respective opposite end surfaces of the optical cavity along an optical path axis of the cavity. The second reflector can alternatively be replaced with a compliant cavity that acts as a Gires-Tournois interferometer. The signal laser is adapted such that acoustic energy incident on the transducer assembly changes the length of a cavity along the optical path axis, or, alternatively, changes the index of refraction in the optical cavity, and such changes result in a substantially linear variation of the optical frequency of light generated by the laser.
In one embodiment, as disclosed in U.S. Pat. No. 5,353,262, the signal laser is mounted in the transducer housing such that acoustic energy incident on the transducer assembly is transmitted along the optical path axis of the optical cavity causing alternating compression and rarefaction of the optical cavity, thereby varying the length of the optical cavity and consequently varying the frequency of light generated by the signal laser. In another embodiment, a compliant cavity that is a Gires-Tournois interferometer is coupled to the lasing medium in lieu of the second reflector such that changes in the incident acoustic energy cause variations in the length of the compliant cavity, which in turn causes a change in the effective optical cavity length of the signal laser with a resultant modulation of the optical frequency of the laser.
In a further alternative embodiment, the signal laser is adapted such that incident acoustic energy is translated into a change in the index of refraction along the optical path between the first and second reflectors such that the optical frequency generated by the signal laser varies in correspondence with the incident acoustic energy. In this embodiment, a piezoelectric device is typically disposed to receive the incident acoustic energy and generate a corresponding electrical signal. The piezoelectric device is electrically coupled to conductors to apply an electric field across the lasing medium. The lasing medium is typically adapted such that variations in the electric field generate a corresponding change in the index of refraction of the lasing medium. Alternatively, the piezoelectric device is coupled to drive an electro-optic device disposed in the optical path and the electro-optic device is adapted to cause a change in the index of refraction.
A laser pump is coupled to the signal laser and is adapted to provide a selectable level of excitation energy to the lasing medium to activate the signal laser. In one embodiment of this invention, the signal laser is mounted in the transducer housing to allow displacement along the optical path axis of a medium in a cavity in the signal laser, the amount of displacement being dependent upon the selected level of excitation energy absorbed by the medium. In a transmit mode, the laser pump is adapted to apply sufficient excitation energy to the medium to generate an ultrasound pulse in an object to which it is coupled.
In an ultrasound system, the optical signal generated by the signal laser is typically coupled to a signal processing assembly for display and analysis. The signal processing assembly advantageously comprises heterodyne detection devices, or alternatively, spectral filter detection devices adapted to generate an output signal corresponding to the amplitude of the incident acoustic energy detected by the signal laser.
Despite the efforts described above, there is still a need to develop improved optical sensors for vibration/ultrasound detection. Advantages of optical sensors over standard piezoelectric sensors include insensitivity to electromagnetic interference (EMI) and the ability to be interfaced with fiber-optic cabling. The former advantage has applications in monitoring vibrations in high EMI environments such as in electrical power plants or along electrical power lines. The latter advantage allows suitably sized sensors to be incorporated within composite structures for process and "in-use" vibration monitoring. Additionally, for medical ultrasonic imaging applications, the final image quality is improved in direct relation to the number of sensors or pixels in an array. Presently, the number of sensors in an array is limited by the size of the cables interconnecting the sensor array to a signal processing console. With fiber-optic cabling, which is substantially smaller than electrical cabling, more sensor array elements can be utilized to provide a simpler, more compact ultrasound detection device. Specifically, in medical imaging and nondestructive evaluation (NDE) of materials for an ultrasound transducer interfaced to support equipment current state-of-the-art ultrasound imaging arrays use a linear array of about one hundred separate ultrasound pixels each of which are connected to a separate coaxial cable. Array sizes are currently limited by the complexity of the interconnect cabling. Thus, it is desirable to increase the number of sensors or pixels in a detection device while at the same time reducing the complexity of the interconnecting cabling. For a practical device, it is also necessary that the detection scheme not add excessive complexity in signal conditioning or in signal processing.